The present disclosure generally relates to document processing devices and methods for operating such devices. More specifically, the present disclosure relates to methods and systems of adjusting the skew of print sheets in a document processing devices.
Sheet registration is performed in document processing devices that require a medium (i.e., the sheet) to be properly positioned and aligned at a known location. A sheet registration system may align and position a sheet in such document processing devices.
FIG. 1A depicts a top view of a portion of an exemplary document processing device known in the art. As shown in FIG. 1A, the document processing device 100 includes three pairs of nips 105a-b, 110a-b, and 115a-b. Transport nips 105a-b are used to transport a sheet to a sheet registration system 120; registration nips 110a-b are used to perform sheet registration; and auxiliary sheet transport nips 115a-b are typically used to move the sheet in the process direction after registration. Although two nips are shown for each nip location, additional or fewer nips may be used. In some cases, additional nips may be used to account for variations in sheet size during the transport or registration processes.
Document processing devices, such as 100, typically have auxiliary sheet transport nips 115a-b between a sheet registration system 120 and a location 125 where sheets need to be registered. The sheet registration system 120 seeks to deliver a sheet to the auxiliary sheet transport nips 115a-b with no more than a certain amount of skew error. However, the auxiliary sheet transport nips 115a-b can introduce additional skew error before the sheet reaches the registration location 125 due to errors and disturbances, such as sheet drag force variation, nip alignment, nip force and other similar factors. In addition, auxiliary sheet transport nips 115a-h are typically driven by a single motor and are not currently used to register a sheet.
The document processing device 100 also includes a plurality of sensors, such as 130a-c, which are used to determine the position and alignment of the sheet prior to, during or after registration. Alternately, encoders can be placed on the nips to assist with sheet registration. Methods and systems using sensors and/or encoders to assist with sheet registration are known to those of ordinary skill in the art.
When using a conventional document processing device, a sheet arrives from an upstream device, such as a feeder, a duplex path or the like. Each set of nips 105a-b, 110a-b, and 115a-b includes, for example, two nips (one on the inboard side and one on the outboard side) that propel the sheet substantially in a process direction. As shown in FIG. 1B, a typical nip, such as 115a, includes a drive wheel, such as 150, and an idler wheel, such as 155. The composition and operation of a drive wheel and an idler wheel are described below.
A normal force caused by loading the idler wheel and friction with the sheet can be used to produce a forward force that propels the sheet. As a result, a lead edge, trail edge or other reference point within the sheet is delivered to the registration location 125 with specified registration performance. In other words, the sheet registration system 120 delivers each sheet at a specified lateral position (x-direction) and angular orientation (skew) at a specified time and with a specified velocity. Other exemplary sheet registration systems are disclosed in U.S. Pat. Nos. 5,697,608 to Williams et al. and 5,094,442 to Kamprath et al.
The sheet registration system 120 passes a sheet to a set of auxiliary sheet transport nips 115a-b that are located between the sheet registration system and the registration location 125. Auxiliary sheet transport nips 115a-b are typically driven by a single motor, which causes the angular velocity of the inboard and outboard nips to be substantially identical. If a variable speed or servo motor is selected, the process direction (x-direction) registration can be finely controlled. However, the use of a single motor precludes fine adjustment in skew in such sheet registration systems. Using separate motors for each auxiliary sheet transport nip 115a-b allows fine skew adjustment to be performed, but at the cost of an additional motor, which can be prohibitive. Moreover, it can be difficult to retrofit a single motor auxiliary nip system to a dual motor configuration.